of the double refraction in strained glass. 487 



Special attention had to be given to the polarizing Nicol. The 

 latter required to be displaced in order that the whole spectrum 

 might be examined. Care had to be taken always to keep it 

 parallel to itself. This was done by keeping the edge of the 

 square stand of the Nicol flat against a fixed straight edge. In 

 this way the motion of the Nicol could introduce no shift of the 

 lines of the spectrum. 



The vertical distances between the black bands were measured 

 by means of a second glass scale which carried a vertical milli- 

 metre scale. This was placed in contact with the other scale, but 

 could be shifted freely to the right and left so as to bring the 

 vertical scale into any required horizontal position. 



6. For the two beams actually used 



Tj = 2-905 cms., t 2 = 2-885 cms., 



a = 152 cms., b = 6 cms. 



The bending moments, after all the corrections had been 

 applied, were found to be 



il/i = 676 kgms. weight cms., 



if 2 = 672 kgms. weight cms. 



The heights of the beams were given by 



Ai = 367 cms., 



h 2 = 3 - 65 cms. 



Whence c u c 2 were calculated to be 



Cj = 164"1 kgms. weight per sq. cm., 



c 2 = 165*9 kgms. weight per sq. cm. 

 and 



p = 340'8 kgms. weight per sq. cm. 



Accordingly, X being known and y [n+1} — y (n) being measured in 

 cms., the absolute value of G can be found in sq. cms. per kgm. 

 weight. 



As what is required, however, is the relative value of C for 

 different colours, and as p remained the same throughout the 

 whole series of experiments, the values of {y { ' l+1) — y {n) )/\ have 

 not been reduced to absolute values by multiplying by p in each 

 case, but the quantity studied has been kept in the form 1/Gp. 



The fringes in the field numbered 2 in the red, increasing to 3 

 in the violet, on either side of the central one. The distances 

 ^d)_y(o) } y (2) — 2/ (0) , 2/ ,:i) — y {0) were measured for a number of 

 sections corresponding to divisions of the horizontal scale. A 



